Photochromism is a property which has been used in the manufacture of light transmissible articles for many years. A compound is said to be photochromic if it changes colour when irradiated and reverts to its original colour when irradiation ceases. The use of photochromics in the manufacture of spectacle lenses is a particular benefit as it enables the efficiency with which radiation is filtered to be varied with the intensity of radiation. Photochromics also have potential for use in a range of other polymeric compositions in products or in applications such as architectural, automotive and aircraft windows and transparencies; inks, paints and coating compositions; optical sensors, filters, switches and data storage devices; and for security and UV exposure sensing applications.
Despite the successful use of photochromic compounds in applications such as lenses there remain limitations which reduce the versatility and potential of this technology.
It is advantageous to control the rate at which photochromic polymeric compositions colour when exposed to radiation and fade on cessation of this exposure. In many situations, it is important to provide rapid colouring and fading kinetics particularly for lenses and spectacles. In some, however, the rate of coloration and fade is slow so that a compromise needs to be made in the components and properties of the substrate to enhance the rate of coloration and fade. For example, many photochromics colour and fade more rapidly in soft materials and yet, for applications such as spectacles, abrasion resistance and hardness are important. This trade off between rate of transformation and hardness produces a dilemma for manufacturers between toughness and photochromic efficiency. In polymeric lenses many photochromics exhibit a slower rate of fade than is desirable. In order to fix photochromics into a matrix it is possible to functionalise a photochromic with an unsaturated group. This results in the photochromic being tethered to the matrix. However, unless the matrix is relatively soft, the rate of fade is adversely affected. Hu et al. Pure Appln. Chem. AA(6) pp 803-810 (1996) also reported that tethering of the photochromic leads to decolouration rate remaining almost constant with increasing dye concentration.
In order to achieve attractive colouring of lenses and/or achieve a good UV barrier photochromic compounds are frequently used in combination. The variation in fade speed between various photochromics makes it difficult to achieve a uniform colour change. As a result the combination of photochromics tends to produce an unattractive colour change on transition to and from the coloured forms. It is desirable to be able to more closely match the fade speed of photochromics.
Our U.S. Pat. No. 7,247,262 describes a photochromic composition containing a photochromic compound and a host matrix. The photochromic compound has a photochromic moiety functionalised to contain one or more pendant oligomer groups. The oligomer groups were believed to provide a nanoenvironment to produce a significant increase in the rate of fade.
The discussion of documents, acts, materials, devices, articles and the like is included in this specification solely for the purpose of providing a context for the present invention. It is not suggested or represented that any or all of these matters formed part of the prior art base or were common general knowledge in the field relevant to the present invention as it existed before the priority date of each claim of this application.